Engaging students in the process of argumentation supports student understanding of scientific knowledge. We present a comparison of arguments from 7th and 11th grade biology students from modeling tasks using an argumentation framework to investigate differences in student approaches during two comparative tasks. While students at both grade levels constructed arguments that contained multiple reasons and epistemic ideals, we identified differences in rhetorical statements and refutational strategies by grade level. High school students attended to the underlying model-evidence relationship to explore differences between competing models and to make comparisons and refute the competing model at higher instances than middle schoolers. Refutational strategies by high schoolers connected multiple pieces of evidence, or made connections between provided models and evidence, while middle schoolers made use of epistemic ideals to refute a competing model. These differences are discussed in conjunction with implications for design.